1. Field of the Invention
The present invention relates to an antenna for use in a wireless communications system. More particularly, the present invention relates to an antenna aesthetically concealed in a sign particularly suited for use in indoor wireless applications.
2. Background of the Invention
In a wireless communications system, such as a cellular telephone system, a network of antennas and radios is typically designed to provide uniform coverage throughout a given service area. A wireless communications provider strives to enable communications at any point over the service area. Since a wireless service area often includes buildings, it is desirable to provide wireless service to customers inside buildings. Unfortunately, indoor wireless coverage is often difficult to maintain. Typically, the quality of indoor coverage is related to the proximity of an outdoor cellular site. For example, a building located adjacent to an outdoor cellular communications site would receive adequate coverage, while a building situated farther away from that site would not. Increasingly, designers are incorporating antennas and radios within buildings to provide indoor coverage. These antennas and radios operate as miniature wireless communications sites within a building.
In order to obtain adequate coverage within a building, it is often necessary to distribute antennas throughout the building. For example, in a multi-story building, it would be necessary to place antennas at strategic locations on every floor in order to obtain sufficient coverage. Unfortunately, antennas for indoor wireless communications are often bulky and unsightly. While it is desirable from a communications standpoint to distribute numerous antennas throughout a building, an architect or building designer may wish to limit the number of antennas for aesthetic purposes.
Currently, numerous antenna manufacturers make antennas for use in indoor settings. These antennas typically come in two different typesxe2x80x94wall mounted and ceiling mounted. For example, the MP8068 series wall-mounted antenna available from Maxrad, Inc. of Hanover Park, Ill. operates at 806 to 960 MHz and occupies a wall surface area of 16.4 inches by 9 inches. The PA002 and PA004 wall-mounted antennas available from Radiall/Larsen of France operate in the 1900 MHz and 850 MHz spectrums, respectively and each occupies a wall surface area of 9.4 inches by 6.75 inches. Likewise, the ASPPD2988 and ASPPK29988 models available from Antenna Specialists of Lorain, Ohio operate at the 850 MHz and 1800 MHz bandwidths, respectively and occupy wall surface areas of 8.5 inches by 8.4 inches and 4.2 inches by 3.9 inches, respectively. The model 7190.01 wall-mounted antenna, available from AlIgon Enterprises of Sweden, operates in the 850 MHz range and occupies a wall surface area of 10.5 inches by 9 inches. In addition, each of these wall-mounted antennas extends out from the wall 1.5 to 3 inches. As is evident, each of these antennas may be unsightly in certain applications, and the placement of these antennas in a building may also be limited. In addition, each of these antennas is a unidirectional antenna. Unidirectional antennas have a transmit and receive area that is located in one direction out from the wall on which the antenna is mounted.
In addition to wall-mounted antennas, many manufacturers make antennas that can be mounted on a ceiling or in a corner. For example, Decibel Products of Dallas, Tex. offers the model DB791 S50N, which operates at 824 MHz to 960 MHz and can be mounted below the ceiling in a corner. This antenna occupies an area of 6.1 inches by 6.1 inches by 6.1 inches. Decibel Products also manufactures an omni-directional ceiling-mount antenna for use in the 850 MHz bandwidth that occupies an area of 4 cubic inches (Decibel Products model number DB784SM5N). In addition, the Decibel Products model number DB781S50N ceiling-mounted bidirectional antenna for use in the 850 MHz spectrum occupies an area of 1 cubic foot. The MPA806 ceiling-mount antenna, available from Antel International of Rockford, Ill., is an omni-directional antenna housed in an elliptical covering with a diameter of 8.5 inches and a height of almost 3.5 inches. The Maxrad Inc. ALPC800 antenna is a quarter wavelength stub antenna for the 800 MHz to 960 MHz frequency range that is disguised in a speaker baffle. This antenna extends almost 3 inches down from the ceiling.
There are numerous problems with each of these conventional antennas. Both wall-mounted and ceiling-mounted antennas can be unsightly. Architects and building designers are reluctant to place large antennas such as those listed within an architecturally pleasing space. Further, each of the conventional antennas listed may only be placed in certain areas within a building. For example, a wall-mounted antenna must be placed, of course, on a wall. The location of a wall within a given space may not provide for the proper placement of an antenna to provide sufficient indoor wireless communications coverage. Likewise, ceiling-mounted antennas may also be constrained as to their placement. For example, a ceiling-mounted antenna must be placed below the ceiling plane. It cannot be placed above the ceiling plane because of limited space and the potential for structures that may block its signal.
In addition, many of the ceiling-mounted antennas manufactured today are omni-directional antennas. In general, in the 800 to 1000 MHz frequency range, manufacturers of ceiling-mounted antennas only offer omni-directional antennas. An omni-directional antenna has a transmit and receive area that is concentrically oriented around the antenna. In this manner, the transmit and receive area is bounded by a sphere or toroid around the antenna. In order to obtain sufficient indoor coverage, it is often necessary to use numerous omni-directional ceiling-mounted antennas. For example, an omni-directional ceiling-mounted antenna may have a transmit and receive area encompassing a circle with a diameter of 20 feet. In such a case, in order to obtain sufficient indoor coverage in a large building, it would be necessary to place numerous omni-directional ceiling-mounted antennas evenly throughout the building. This may not be possible because of certain design constraints and also because the number of antennas would be unsightly. In addition, in order to provide sufficient indoor coverage with omni-directional antennas, it may be necessary to place antennas such that their coverage area extends outside the building. This bleed-out of antenna coverage is undesirable and inefficient. When using omni-directional antennas, the designer of an indoor wireless antenna system often faces a tradeoff between indoor coverage and bleed out.
On the other hand, directional antennas provide better coverage while eliminating the bleed-out problem. Directional antennas generally have a transmit and receive pattern that is better suited to indoor design. Further, fewer directional antennas would be required to cover the same amount of interior space. However, directional antennas tend to be larger than omni-directional antennas. In order to focus the energy in a directional antenna, it is necessary to increase the size of that antenna. For example, a unidirectional antenna or a bi-directional antenna for any given frequency range is typically larger than an omni-directional antenna. A unidirectional antenna has a transmit and receive area that is disposed in one direction from the antenna, while a bidirectional antenna has a transmit and receive area that is disposed in two opposite directions, one from each side of the antenna. In addition, the larger the antenna size, the more efficient the antenna. A larger size antenna has more capability to direct its power in a certain direction. Therefore, the more efficient antennas, and the antennas that would be better suited for providing sufficient indoor coverage, are also the larger more unsightly antennas. For example, a unidirectional or bi-directional patch antenna operating in the 800 to 1000 MHz frequency range would occupy an area of approximately 6 by 6 inches. Likewise, the same antenna operating at 1900 MHz would be about half that size or 3 by 3 inches.
As is commonly known, the size of an antenna is directly proportional to the wavelength of the signal with which it operates. In this manner, the wavelength is proportional to the inverse of the frequency. A signal with a larger wavelength (and a smaller frequency) requires a larger antenna. Conversely, a signal with a smaller wavelength (and a larger frequency) requires a smaller antenna.
Further, it is a challenge to provide indoor coverage because buildings are of different shapes and sizes. For example, many new office buildings are not simply square but often are rectangular or elongated or have sweeping curves. In order to design an 850 MHz antenna system to provide sufficient indoor coverage in an odd-shaped building, it is often necessary to use numerous omni-directional antennas. Since manufacturers typically only make omni-directional antennas for the 850 MHz bandwidth, many of the problems previously described are encountered in designing an antenna system for an odd-shaped building. For example, in order to provide proper coverage, it is necessary to use antennas such that bleed-out occurs outside the building. In addition, due to placement constraints, it may not be possible to provide adequate indoor coverage in an odd-shaped building.
Indoor coverage is also desirable in noisy environments. In general, a wireless communications signal must have a minimum signal strength of 20 decibels (dB) higher than the noise on that particular channel. Many urban areas are congested with different types of communications systems. These different communications systems may provide a noisy environment in which a wireless system operates. Therefore, it is desirable to have antennas disposed throughout a building in order to provide adequate signal strength for wireless communication.
Further, a wireless communications device, such as a cellular phone, may interfere with various equipment. For example, in a hospital, it is often forbidden to use a cellular phone. Many cellular phones interfere with the various electronic devices used in a hospital. In general, a cellular phone has a maximum power output of between 400 and 600 milliWatts (mW). A cellular telephone operates at the power necessary in order to receive a signal from a cellular site. For example, a cellular telephone that is located a far distance from a cellular communications site will operate at its maximum power, 600 mW. Conversely, a cellular telephone located close to a cellular communications site will output a lot less power, for example, 50 mW. This lower power signal is less likely to interfere with electronic equipment in a hospital. In addition, this lower power signal also preserves the battery life of a cellular phone. Therefore, it would be desirable to place numerous antennas throughout a hospital so as to avoid interference with electronic equipment. In addition, it is desirable to place numerous antennas throughout any building to preserve battery life on a wireless device.
In one aspect consistent with the principles of the present invention, a concealed antenna assembly includes a base aligned with and attached to a ceiling, a sign having two transparent planar pieces disposed substantially parallel to each other, the two planar pieces of substantially the same size and shape, the two planar pieces disposed substantially perpendicular to and beneath the base, the two planar pieces each having a front and back face, the two planar pieces arranged so that their back faces face each other, the back faces at least partially defining a channel, a substantially planar microstrip antenna concealed in the channel, the antenna having a front and back face and a radio frequency connection point, the front or back face of the antenna having lettering visible through at least one of the planar pieces of the sign, wherein a surface area of the front or back face of the antenna is equal to less than a surface area of the front or back face of the planar pieces, and a light source for illuminating the sign.
In another embodiment consistent with the principles of the present invention, a concealed antenna assembly includes a base aligned with and attached to a ceiling, a planar sign having a transparent front and back face and an interior surface, the sign disposed substantially perpendicular to and beneath the base, the interior surface defining a channel extending within the sign, a substantially planar microstrip antenna disposed in the channel, the antenna having a front and back face and a radio frequency connection point, the front or back face of the antenna having lettering visible through the front or back face of the sign, wherein a surface area of the front or back face of the antenna is equal to or less than a surface area of the front or back face of the sign, and a light source for illuminating the sign.
In yet another aspect consistent with the principles of the present invention, a concealed antenna assembly includes a base aligned with and attached to a ceiling, a planar sign having a front and back face, the planar sign having lettering, the planar sign disposed substantially perpendicular to and beneath the base, a substantially planar microstrip antenna having a front and back face and a radio frequency connection point, the antenna disposed on the front or back face of the sign such that the front or back face of the antenna is adjacent to the front or back face of the sign, wherein a surface area of the front or back face of the antenna is equal or less than a surface area of the front or back face of the sign, and a light sources for illuminating the sign.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.